Field of the Invention
The present invention relates to a radiographic apparatus, and more particularly, to a radiographic apparatus that includes a radiation sensor.
Description of the Related Art
Radiographic apparatuses that generate charge in detection elements in accordance with a dose of emitted X-rays and read the generated charge as image data are being developed. A radiographic apparatus of this type is known as an FPD (Flat Panel Detector), and is conventionally formed as a special-purpose radiographic apparatus (also called an anchored type, for example) that is integrated with a supporting base or the like. In recent years, however, a radiographic apparatus of a portable type (also called a cassette type, for example) that has detection elements and the like placed in a housing and can be carried around has been developed and put into practical use.
Conventionally, such a radiographic apparatus constructs an interface with an X-ray generator, and exchanges signals and the like with the X-ray generator. When the preparation in the radiographic apparatus is completed, X-rays are emitted from the X-ray generator to the radiographic apparatus via a subject, and imaging is performed. In a case where the radiographic apparatus and the X-ray generator were manufactured by different makers, however, it is not necessarily easy to construct an interface between the two, and an interface cannot be constructed in some cases. If an interface cannot be constructed (or is not constructed) between the radiographic apparatus and the X-ray generator, the problems described below will be caused, for example.
Prior to imaging, a radiographic apparatus normally performs a reset process to remove the remaining charge from the respective detection elements. If there is no interface constructed as described above, the radiographic apparatus continues to perform the reset process on the detection elements, without noticing that X-rays have been emitted from the X-ray generator. As a result, the charge generated in the detection elements through the X-ray emission is removed from the detection elements by the reset process.
If such a situation is caused, the X-ray emission from the X-ray generator is wasted, and the X-ray supply for the X-ray generator is exhausted for nothing. Furthermore, X-rays need to be again emitted from the X-ray generator, and imaging needs to be again performed (retaking). As a result, the patient as the subject is exposed to a larger dose, and further burden is put on the patient.
In view of this, a radiation sensor is attached to a radiographic apparatus, so that the radiographic apparatus detects a start of X-ray emission based on an output value from the radiation sensor. In such a case, when detecting a start of X-ray emission, the radiographic apparatus stops performing the reset process on the detection elements, puts the switching elements of the respective detection elements into an off-sate, and causes the detection elements to transit to a charge accumulating state in which the charge generated in the detection elements through X-ray emission is accumulated in the detection elements.
In a case where a radiation sensor is attached to a radiographic apparatus as described above, however, the radiation sensor senses cosmic rays, and the radiographic apparatus might wrongly detect a start of X-ray emission based on the information about the cosmic rays. In view of this, JP 4881796 B1 discloses a technique by which the position of a radiographic apparatus prior to imaging is adjusted so that the normal line of the detector plane of the radiation sensor attached to the radiographic apparatus extends substantially in the horizontal direction. By this technique, the probability that cosmic rays enter the radiation sensor is reduced.
Meanwhile, a radiographic apparatus disclosed in JP 4763655 B1 compares two or more pieces of image data, and determines whether there is any influence of an external radiation component that is not X-rays emitted from an X-ray generator. If there is some influence, the influence is removed.
In a case where the imaging method disclosed in JP 4881796 B1 is employed, the position of the radiographic apparatus to be used for imaging is limited. A radiographic apparatus of a portable type (a cassette type) can be inserted between the body of a patient and a bed, for example. A radiographic apparatus of a special-purpose type (an anchored type) does not have such an advantageous feature. In a case where a radiographic apparatus of a portable type is used, the normal line of the radiation sensor extends substantially in the vertical line. In a case where a radiographic apparatus of a special-purpose type is used as a radiographic apparatus for so-called recumbent-position imaging, and performs imaging by emitting X-rays from above while the patient is lying on a table, the normal line of the detector plane of the radiation sensor also extends substantially in the vertical line. Where the imaging method disclosed in JP 4881796 B1 is employed, however, the above described imaging with a radiographic apparatus cannot be performed.
In a case where the method disclosed in JP 4763655 B1 is employed, it is only after image data is read out that a check can be made to determine whether there is some influence of an external radiation component. In a case where X-ray emission to a radiographic apparatus is detected based on an output value of a radiation sensor as described above, immediacy is required so as to instantly determine whether the cause of a value output from the radiation sensor is X-ray emission or external radiation, and detect X-ray emission, instead of external radiation. By the method disclosed in JP 4763655 B1, however, it is only after image data is read out that a check can be made to determine whether X-rays are emitted to the radiographic apparatus or whether external radiation is emitted to the radiographic apparatus. This timing is too late.
In the description below, radiation other than X-rays emitted from an X-ray generator, including the cosmic rays disclosed in JP 4881796 B1 and the external radiation disclosed in JP 4763655 B1, will be collectively referred to as natural radiation. Natural radiation includes not only the above mentioned cosmic rays and nature-derived radiation that is emitted from radioactive elements and the like existing in the surface of the ground and under the ground, but also radiation from radiotherapeutic agents and radiotherapeutic test agents existing in facilities such as a hospital, and radiation from radioactive materials derived from artificial nuclear fuel and the like scattered or leaking from a nuclear power plant and the like.
In terms of wavelengths, natural radiation includes not only X-rays but also radiation having wavelengths that exceed the wavelength range of X-rays, such as gamma rays. In this specification, when X-rays emitted from an X-ray generator are not distinguished from natural radiation, any radioactive rays will be referred to simply as radiation.